It is common knowledge for a person skilled in the art that the design of an engine, say, an internal combustion engine, involves taking account of the normal conditions in which the engine operates. For example, the designer must know beforehand what will be the speed, i.e. the number of revolutions per minute, at which the engine is likely to be run for the greater part of the time.
In the case of high performance engines, operation will be mostly at maximum revs, whereas the engine of a standard production vehicle will be utilized at all speeds across its specified range, with the greater part of the time spent at a running speed (rev/min) somewhere between idling and maximum. This difference in requirements alone will dictate differences in the size and shape of the inlet and exhaust valves, and of their relative passages and lift cams.
High performance engines require valves and passages of given dimensions, and the valves must be able to remain open for relatively long intervals of time. With the high running speed of these engines, the air-fuel mixture and the exhaust gases generate such high levels of kinetic energy that mixture continues to enter the cylinder, and exhaust gases to exit, even during the compression and induction strokes, respectively. This action is also favored by the limited velocity of the piston at its top and bottom dead centers.
Conversely, when a high performance engine runs at low revs, the fluid, whether fuel mixture or exhaust gas, develops insufficient kinetic energy to offset the movement of the piston, and is thrust back into the relative passage, the result being a loss of volumetric efficiency. Volumetric efficiency is the ratio between the effective weight of fuel mixture admitted into the cylinder per unit of time, and that which would in theory fill the swept volume in the same unit of time at s.t.p., that is, with the identical cylinder temperature and inlet pressure conditions. In short, volumetric efficiency provides an index to the cylinder's correct replenishment. The current state of the art admits of proportioning the valves and passages and the cams that control the opening and closing movements of the valves, according to operating conditions, so as to obtain given volumetric efficiency, maximum output torque and power characteristics; however, such proportions are essentially fixed, and can only be altered by replacing or modifying the parts in question.
Volumetric efficiency can be varied by modifying the design of the inlet and exhaust passages and setting the opening and closing time lapses of the valves to given durations. Modifying the design of the inlet and exhaust passages necessarily involves altering the dimensions of the valves, and accordingly, extra power can be extracted from an engine by enlarging the valves and thus increasing the amount of fluid that enters or leaves the cylinder per unit of time. Such a step produces increased volumetric efficiency and higher maximum output torque, but also dictates that maximum torque, and maximum power, will occur at higher respective running speeds.
On the practical level, this type of modification involves removing and machining the cylinder head, and refitting it with the bigger valves mentioned. Such modifications are quite simple to implement, but are costly and signify immobilizing the vehicle for some considerable time. Even a modification of the amount of cam lift involves a lengthy standstill in the workshop for replacement, at very least, of the camshaft.
Conventionally then, engines are designed to power and torque specifications in which maximum output occurs at the running speed likely to be reached for the greater part of the time when in use. Clearly, this signifies that optimum volumetric efficiency is unobtainable at low and high running speeds, and the same must also apply for general performance.
In practice, the higher the maximum power and torque output specifications, the higher will be the speed at which the engine has to run; similarly, the lower the running speeds at which maximum power and torque are generated, the harder it becomes for the engine to reach high running speeds.
Accordingly, the object of the present invention is to overcome the drawbacks mentioned, and to permit of varying the power and torque characteristics of an engine swiftly and economically.